The present invention relates generally to a method and an apparatus for controlling a speed ratio of a continuously variable transmission for a vehicle, and more particularly to improvements in such method and apparatus for increased fuel economy of the vehicle during acceleration periods.
In the art of a continuously variable transmission for a vehicle to transmit a rotary motion of an engine to drive wheels with a continuously variable ratio, a control apparatus is known which comprises regulating means for controlling the speed ratio of the variable transmission (ratio of a rotating speed of its output shaft to a rotating speed of its input shaft) such that an actual speed of the engine coincides with a target speed thereof which is determined based on a currently required output of the engine and according to a predetermined relation between the target speed and the required output of the engine. With such a known control apparatus to control the speed ratio of a variable transmission, the actual engine speed is controlled into conformity with a target engine speed which is continuously or intermittently determined so that the specific-fuel consumption is minimum. Thus, the control apparatus permits the transmission and the engine to operate with a high fuel economy over an entire range of the vehicle speed, particularly while the vehicle is running in a relatively stable conditions or at a relatively constant speed. However, when the vehicle is accelerated at a high rate, particularly when it is desired to achieve a quick acceleration of the vehicle with an abrupt depression of an accelerator pedal in a relatively large amount, for example, all the way to its full-speed or full-stroke position, the target speed of the engine is rapidly increased to a level corresponding to the amount of depression of the accelerator pedal (representative of the required output of the engine). As a result, the speed ratio of the variable transmission is changed or controlled so that the actual engine speed is rapidly increased to keep up with the rapidly increasing target speed. Hence, the engine tends to operate with increased noises, and the driving comfort of the vehicle is reduced. Further, it is generally recognized that the transmission efficiency is unavoidably decreased in a process of change in the speed ratio of the variable transmission. This means that a rapid change in the speed ratio during acceleration of the vehicle causes a reduction in the transmission efficiency, and consequently a decrease in the fuel economy of the vehicle. In short, the known apparatus for controlling a continuously or steplessly variable transmission suffers the problems of high operating noises and relatively low fuel economy of the engine.